Generally, an automotive air conditioning apparatus includes an air conditioning duct which is open at one end in the interior of the vehicle and at the other end into atmospheric air. A heat exchanger is disposed in the duct, and a fan is provided for supplying air through the duct from the opening at one end into the vehicle interior. During a heating operation, a stream of air to be supplied from the one end opening into the vehicle interior is formed within the duct on the basis of the driving force applied to the fan, so that heat is imparted from the heat exchanger to the air, and the resulting high temperature air is utilized to heat the vehicle interior.
For use as a heating apparatus including the aforesaid heat exchanger wherein a heating medium is circulated through the heat exchanger, a heat pump system and a forced water supply system are known. In the heat pump system, the refrigerant is compressed by a motor-powered compressor so that heat is accumulated in the refrigerant. Next, the refrigerant is supplied to the heat exchanger disposed within the duct, to thereby heat the air flowing within the duct. Then, the refrigerant, after having been expanded by an expansion valve, is fed to the heat exchanger in which the refrigerant absorbs heat from the outside air. In the forced water supply system, water as a heating medium is first heated by a heater. In turn, the heated water is fed by means of a circulating pump to a heat exchanger disposed within the duct wherein the heated water heats the air flowing in the duct.
A known air conditioning apparatus includes a heat pump system and a forced water supply system so as to be able to supply heated air into the vehicle interior promptly upon starting of heating operation. In such an air conditioning apparatus, a heat exchanger of the heat pump system and a heat exchanger of the forced water supply system are disposed within the air conditioning duct so that the air fed in by means of a fan provided also in the air conditioning duct is heated by the two heat exchangers in the duct and supplied into the vehicle interior.
The quantity of air to be heated by the heat exchanger of the forced water supply system is usually adjusted by means of a mixing valve disposed in the duct. For example, in the air conditioning apparatus disclosed in Japanese Patent Laid-Open Publication No. 7-323717, the position of a mixing valve, or valve travel is adjusted according to the temperature preset by an operator so that the quantity of air to be heated in the forced water supply system can be controlled accordingly and so that the temperature of the water flowing in the heat exchanger of the forced water supply system is variable according to the preset temperature. Specifically, according to the teaching of the above cited disclosure, in proportion as the preset temperature is made higher, the travel of the mixing valve is increased so that a greater quantity of air is brought in contact with the heat exchanger of the forced water supply system and so that the temperature of the water is raised, whereby the temperature of the air supplied into the vehicle interior is promptly raised.
When the heat pump system is compared with the forced water supply system, generally it may be said that the heat pump system has higher heating efficiency but its heating capability under very low temperature conditions (e.g., -5.degree. C.) is low. In contrast to this, the forced water supply system has no such high heating efficiency as the heat pump system, but involves no appreciable drop in its heating capability under very low temperature conditions. Therefore, an air conditioning apparatus equipped with the two systems can operate according to the ambient conditions (e.g., outdoor air temperature) in such a way that the heat pump system only is operated at one time, while at other times the heat pump system and the forced water supply system are operated in combination.
However, where the heat pump system and the forced water supply system are used in combination, it is necessary to control the temperature of the refrigerant flowing in the heat exchanger of the heat pump system and the temperature of the heating medium flowing in the heating exchanger of the forced water supply system. For example, assume that the heat exchanger of the heat pump system is disposed on the upstream side of the air flow in the duct, while the heat exchanger of the forced water supply system is disposed on the downstream side of the air flow, and that the temperature of the refrigerant in the heat exchanger of the heat pump system is 60.degree. C. and the temperature of the heating medium in the heat exchanger of the forced water supply system is 40.degree. C. In that case, the air heated at the heat exchanger of the heat pump would contact the heat exchanger of the forced water supply system, at a location downstream of the air flow, with the result that the heated air would be deprived of heat. As such, any sufficiently heated air could not be supplied into the vehicle interior, and this would result in wasteful power consumption.
Another problem is that where ambient conditions (temperature conditions) require operation of the heat pump system only, if the forced water supply system is stopped upon start of heating operation of the heat pump system, the air heated by the heat exchanger of the heat pump system contacts the heat exchanger of the forced water supply system which is held at a low temperature, so that the latter mentioned heat exchanger is deprived of its heat. Therefore, it takes time to raise the temperature of the air blowing out from a vent register.
A further problem is that if the first heat exchanger of the heat pump system frosts, the endothermic efficiency of the refrigerant is lowered.
Additionally, when the heat pump is started under severe ambient conditions, e.g., at a very low ambient temperature, a large load may be exerted upon the compressor. Under such conditions, therefore, the heat pump may be kept from being started in order to avoid such overloading; and this will result in a considerable drop in the heating capability of the air conditioning apparatus.